There is known an image forming apparatus provided with an optical scanning device having first and second light sources, a polygon mirror that deflects first and second beams emitted from the respective first and second light sources, a first scanning optical system disposed on one side of the polygon mirror and forms the image of the first beam on a first photosensitive body, and a second scanning optical system disposed on the other side of the polygon mirror and forms the image of the second beam on a second photosensitive body. In the disclosed technology, an optical sensor for detecting the first beam is provided on the upstream side of the first scanning optical system in the scanning direction, while no optical sensor is provided on the second scanning optical system.